This invention relates to the conveying of particulate material suspended or entrained in a pressurised gas and conveyed with the gas along a duct.
The invention has particular application to pneumatic conveying of powdery particulate material although there will be applications in which the particulate material to be conveyed is not strictly in the form of a powder and/or in which the conveying gas may be a gas other than air.
One particular application of the invention is in the field of xe2x80x9cDense Phasexe2x80x9d conveying in which compressed air at relatively high pressure of the order of two bar or more is applied to a pressure vessel charged with a batch of material to be conveyed and the material is conveyed along a duct extending from the pressure vessel to a discharge outlet from the duct, usually into a discharge vessel. Conveying rates in such systems vary widely, commonly falling in the range 5 to 50 tonne/hr but can be anything from under 1 tonne/hr to over 1000 tonne/hr. Conveying distances can vary between a few hundred meters to several kilometers.
Dense Phase pneumatic conveying is used for conveying a wide range of powdery or fine grained materials such as mineral powders, fine granular aluminium, cement, quick lime, coal dust, flour and other food powders, and pharmaceutical powders. Some of these materials can be difficult to handle in that they cause abrasive damage to the conveying duct and the material itself can suffer severe and detrimental attrition in passing through the duct. It is therefore important to control the peak velocity of the material passing through the conveyor duct. In a conventional system the pressure along the duct steadily reduces at a constant rate throughout the length of the duct to fall to atmospheric pressure at the discharge end of the duct. Accordingly, if the duct is a simple cylindrical pipe of constant diameter, the velocity of the material flowing through the duct will progressively rise as the pressure declines and this can result in extremely high velocities in the pipe. It is therefore normal to provide a duct in the form of stepped piping of successively increasing diameter so as to produce successive step-wise reductions in velocity of the material along the pipe. This adds to the complexity of the system, requiring careful design, fabrication and installation. Particularly in long distance applications it can also result in a need for very large diameter pipes. All of these factors produce sufficiently increased costs. Moreover, the velocities of material in the duct can still be excessive and with many materials, duct abrasion and material attrition remains a problem. By the present invention these problems can be very significantly reduced and the fabrication of the conveyor ducting can be very much simplified.
According to the invention there is provided a method of conveying particulate material comprising:
locating a quantity of particulate material to be conveyed within a pressure vessel;
pressurising the pressure vessel with pressurised gas;
directing pressurised gas and entrained particulate material from the pressure vessel into a conveyor duct through which the material is conveyed to a discharge location; and
discharging the conveyed particulate material from the duct at the discharge location;
wherein the flow of gas and entrained particulate material discharging from the duct is subjected to constriction producing a discrete pressure drop of at least 5 kPa.
Preferably, the conveyed material is discharged from the duct into a discharge vessel at said discharge location, the pressure within the discharge end of the duct upstream of the restriction being at least 5 kPa greater than the pressure within the discharge vessel.
The pressurised gas may be pressurised air and the interior of the discharge vessel may be at atmospheric pressure.
Preferably, the pressure drop caused by the restriction of the discharge flow is in the range 100 to 200 kPa.
The pressure vessel may be pressurised to a pressure of at least 3 bar.
The particulate material may be conveyed through said duct through a distance in the range 50 meters to 5 km, although even greater distances are feasible.
The invention also provides conveyor apparatus for conveying particulate material, comprising:
a pressure vessel to receive a charge of particulate material to be conveyed;
pressurising means to pressurise the pressure vessel with pressurised gas;
a conveyor duct having an inlet connected to the pressure vessel to receive pressurised gas and entrained particulate material from the pressure vessel and extending to a discharge end of the duct; and
a flow restrictor at the discharge end of the duct to present a discrete reduction of at least 20% in the effective cross-sectional area for flow from the discharge end of the duct.
Preferably, the discharge end of the duct is connected to a discharge vessel to receive the discharging flow of gas and conveyed particulate material.
The pressurising means may comprise means to admit pressurised air into the pressure vessel. That means may comprise an air compressor providing a source of the pressurised air.
The duct may be in the form of a tubular pipe.
The restrictor may be in the form of an orifice plate installed in the discharge end of the duct and provided with an orifice providing the reduced cross-sectional area for discharge flow.
The orifice plate may, for example, be an annular plate defining a circular central orifice.